Newman Atkinson says:

David, Digitrax has a chip with or without sound that is made for your particular Athearn Engine.

You can check out Digitrax.com and they have a chart to cover each engine and what they use

Tusnomi (Sorry Spelling) Also has charts to pick out what is best for your athearn engine.
I know that the Digitrax has a chip and harness that the wires will clip right on the wiring for your engine. That is not to say that most Athearn engine motors must be raised off the frame and insulated from the frame Usually a good electric tape will do. Also the harness will come with wiring for lights. You MUST remove the bulb from the current bulb support and is continuing to use the standard bulb must insulate the bulb socket from the support stand. You will no longer use the support stand for the ground from the bulb. Instead you will wire the white wire to that bulb positive side and if using a rear bulb then do the same with the yellow wire to that bulb. Since the bulbs can no longer go to case ground as before the harness has a blue wire called a common and that is the wire that all bulbs will feed to as your ground wire. If you leave the ground side of the bulbs to the case ground as they were initially wired you will experience problems. If this is your first engine to wire in a chip then use this as a your wiring requirement whether you use digitrax or any other brands. The harnesses are the same ” Quote: All chips being installed no matter what power wires must go straight to the chip on the red and black wires no exception and everything the chip feed including motor, headlights, ditch, lights, or sound must feed to and from the chip. Nothing goes straight to chassis ground. No short cuts. Every thing power from the track must go straight to the chip everything to anything else the bulb and motor feeds must go to the chip. A couple more things Athearn has a contact clip that makes contact with the frame under the motor. If you are using a chip with the new clip wired to the harness carefully remove the old clip without the spring flying off from the brush and install the new clips onto the motor. Usually the orange and gray wires. Be sure to insulate the motor (lower clip) from the frame with electric tape before reinstalling the motor. Most cases the long spring clip that clips to the top motor contact and goes to both post contacts on the wheel trucks are removed and is replaced with the red wire to both contact posts. (make sure it is a flexible wire to to allow the truck to float. A stiff wire will hinder the truck from turning.)
Just remember that (No. 1) power from the tracks and wheels feed straight to the chip both positive and negative side (red and black wires) 2. Remove all contacts that go to ground or connect to the frame Positive or negative sides. 3. All lights and motor feed straight to the item it is powering both positive and negative side (Use the common blue wire for your return for all lights)
back to the chip. Sound is just another addition to things you can add and those wires should all be part of the harness. The last thing is all chips come on standard code 03 addresses so after you get it up and going then you will want to switch the address to the one you want for the engine (most use the engine number) All motors are different so a tweak of the speeds will be necessary to run even with other engines at the same speed settings. (you may need to have someone help you on your first few engines. Last item: It is suggested that you should update your bulbs to LEDs while making this change. Since DCC is a bit higher voltage you will most likely burn your regular bulbs out quicker. Be sure to use resistors in with your lighting circuits to each bulb and maybe to your regular bulbs. For your LEDs your hobby shop should have the LEDs with the resistors in the package. Don’t run the LEDs without the resistors. Hope I didn’t scare you. No shortcuts to ground and all wire go to the chip first no matter what chip brand you use and insulate the motor.
from Newman Atkinson